I need help selecting a transistor

[red913]

So I've used MOSFET's before but it looks like basic BJT transistors are a lot cheaper. So I would like to figure out how to select one of them. My basic project is a 4 digit 7 segment display, common anode. So each of the 4 digits get's it's own supply and all the individual segments get grounded, through a 220ohm resistor, to turn on. I checked with my DMM and each of the single digits can pull close to 60mA. Since I'll be multiplexing they'll need to turn on and off at least 50 times a second. So where do I go from here in selecting a transistor? What other information do I need to do this? I'm thinking I'll be using a 9V battery to power this project with a 5V regulator. So the transistor will probably switch the same amount of current it will be feeding,+5V.

[larryd]

Look at ULN2803 to see if it fits your needs.

[cjdelphi]

Any npn will do... I can't think of a transistor which can't handle less than 20ma...

[JimboZA]

The table halfway down this page might help.

[Paul__B]

Sounds to me as if you want a MAX7219.

It does have an internal 7 segment decoder for common cathode displays, but there is no need to use it even with common cathode displays, indeed you cannot if you want digits other than 0 to 9 and H,E,L,P.

It certainly makes things easier than all the wiring you would be using otherwise.

Should you nevertheless choose to use transistors and given that they are red LED displays, you can use NPN BC547-like transistors in emitter follower mode with no base resistors and their collectors connected to the 9V supply bypassing the regulator.

[PaulRB]

Look if ULN2803 to see if it fits your needs.

No good with common anode display. red913 needs a high-side switch for each digit, if I understand/guess correctly.

Normally a high-side switch (meaning between the positive supply and the component being switched) is done with a pnp transistor (for example bc327). A base resistor would be needed eg. 4K7.

Paul__B's suggestion of using an npn as an emitter follower also works and has the advantage of not needing a base resistor (the base current is naturally limited in this configuration).

The disadvantage of npn as emitter follower is that it drops quite a lot of the supply voltage (0.7V or more). As Paul__B says, with red or green displays, which have a low forward voltage (around 2~2.5V), this won't be a problem.

With blue or white displays (rare) the forward voltage is higher (up to 3.5V) and they may not work with the emitter followers, as there might not be quite enough remaining voltage from 5V to drive them, even with very low series resistors (must not leave those out!). The pnp would have an advantage here because they will only drop around 0.3V.

If you connect the npn to 9V as Paul__B suggested, you won't have any more voltage than if you connected to 5V, so still can't run  blue/white displays. (For that you would have to use PNPs as high-side switches, with NPNs to switch the PNPs on.) But at least you could run your displays directly from the 9V supply, avoiding that load on the Arduino's regulator.

Previous paragraph heavily edited following advice and corrections from Paul__B!

Paul

[Paul__B]

If you connect the NPN to 9V as Paul__B suggested, you will still have plenty of volts left, so no problem even with blue/white displays. But don't forget to recalculate your series resistors for 9V rather than 5V!

May I suggest you re-think your advice here?

[fungus]

So I've used MOSFET's before but it looks like basic BJT transistors are a lot cheaper. So I would like to figure out how to select one of them. My basic project is a 4 digit 7 segment display, common anode. So each of the 4 digits get's it's own supply and all the individual segments get grounded, through a 220ohm resistor, to turn on. I checked with my DMM and each of the single digits can pull close to 60mA. Since I'll be multiplexing they'll need to turn on and off at least 50 times a second. So where do I go from here in selecting a transistor? What other information do I need to do this? I'm thinking I'll be using a 9V battery to power this project with a 5V regulator. So the transistor will probably switch the same amount of current it will be feeding,+5V.

No problem. Pretty much any BJT will do all of that (and much more).

[PaulRB]

If you connect the NPN to 9V as Paul__B suggested, you will still have plenty of volts left, so no problem even with blue/white displays. But don't forget to recalculate your series resistors for 9V rather than 5V!

May I suggest you re-think your advice here?

Unreserved apologies for any confusion I have caused! I will edit my post and fix before any more confusion happens. You will have to give me a little clue. I have re-read your message any my own but I'm not seeing it. (If you hear some strange noises after, that will probably be me kicking myself)

[Paul__B]

An emitter follower - using a bipolar transistor, in this case NPN - pulls the emitter up to V_be (about 0.7V) less than the base.  The collector can if necessary, be pulled down to V_cesat less than 0.7V to effect this or more practically, you could pull the emitter up very close to the V_cc by pulling the base up above V_cc.  Of course, you are rarely going to do that (except on the high side of a Totem Pole power converter switch as you must do using FETs).

The point is that on a 5V supply, presuming the chip output will pull up to nearly 5V, the emitter can be pulled up to nearly 4.3V - less actually as the chip output - when driving current - can not pull up to nearly 5V.  Since it is in fact, an emitter follower, it makes almost no difference whether the collector is supplied from 5V or 9V.

So you do not re-adjust the current control resistors - which would be on the cathode side of the LEDs - for a 9V collector supply.

The reason why it is such a good idea to power the collectors from 9V is that you use the individual transistors to buffer the current rather than the 5V regulator.

[Pelleplutt]

Schematics ALLWAYS says more than 1000 words

[PaulRB]

The point is that on a 5V supply, presuming the chip output will pull up to nearly 5V, the emitter can be pulled up to nearly 4.3V - less actually as the chip output - when driving current - can not pull up to nearly 5V.  Since it is in fact, an emitter follower, it makes almost no difference whether the collector is supplied from 5V or 9V.

I actually did no know that, although it made sense as soon as I read it. Many thanks Paul__B. I have amended the offending paragraph - please review it again for me?

[PaulRB]

Schematics ALLWAYS says more than 1000 words

Only if you can read the schematic...

[pwillard]

And.... sadly, the schematic is sort of wrong.  You do not put the current limiting resistors on the common display pins, you place the resistors on the individual  display 'segment" pins.  If you don't do this, when you have only 2 segments lit to display a "1", each led gets gets 1/2 the current from R and if you display an "8", all segments... each only gets an 8th of the current.  So, with 330 ohms, and displaying a "1" each led gets ~5mA..  With an '8'  on the LED each gets ~1mA.

And then you will be coming here and saying... why are my LED's so dim?

[DrAzzy]

A UDN2981 might be suitable as a high-side driver?

It's like a ULN2803, only it drives the high side.